Process for purifying ApoA or ApoE

ABSTRACT

A process for purifying apolipoprotein A (ApoA) or apolipoprotein E (ApoE), or variants or mixtures thereof, comprises contacting a first aqueous solution comprising ApoA or ApoE and endotoxins with a matrix comprising an immobilized compound with an end group comprising two or three nitrogen atoms bonded to a carbon atom for attaching the endotoxins to the matrix, and subsequently treating the matrix comprising the immobilized compound with a second aqueous solution comprising a surfactant for releasing the ApoA or ApoE while the endotoxins remain attached to the matrix. Alternatively, the process comprises contacting the first aqueous solution comprising ApoA or ApoE and endotoxins with an anion-exchange matrix for attaching the ApoA and ApoE to the matrix, treating the anion-exchange matrix with a second aqueous solution comprising a compound comprising two or three nitrogen atoms bonded to a carbon atom for releasing the endotoxins while the ApoA or ApoE remains attached to the matrix, and then releasing the ApoA or ApoE from the matrix.

FIELD OF THE INVENTION

The present invention relates to a substantially endotoxin-free apolipoprotein A (ApoA) or apolipoprotein E (ApoE) and a process for producing the same, by separating the endotoxins from the ApoA or ApoE, or variants or mixtures thereof, by contacting a first aqueous solution containing said ApoA or ApoE with a matrix containing an immobilized compound with an end group comprising two or three nitrogen atoms bonded to a carbon atom, and subsequently treating the matrix containing an immobilized compound with a second aqueous solution containing a surfactant, or by contacting a first aqueous solution containing said ApoA or ApoE with an anion-exchange matrix, and subsequently treating the anion-exchange matrix with a second aqueous solution containing a compound comprising two or three nitrogen atoms bonded to a carbon atom. The invention further relates to use of a matrix containing an immobilized compound comprising two or three nitrogen atoms bonded to a carbon atom and a solution containing a surfactant, or an anion-exchange matrix and a solution containing a compound comprising two or three nitrogen atoms bonded to a carbon atom, for removing endotoxins from aqueous solutions containing ApoA or ApoE, or variants or mixtures thereof. The thus produced ApoA or ApoE can be used for the manufacture of a medicament in the treatment of atherosclerosis and cardiovascular diseases, as well as in a method for treatment of atherosclerosis and cardiovascular diseases when administered in a therapeutically effective amount.

BACKGROUND OF THE INVENTION

The clear correlation between elevated levels of serum cholesterol and the development of coronary heart disease (CHD) has been repeatedly confirmed, based on epidemiological and longitudinal studies. The definition, however, of complex mechanisms of cholesterol transport in plasma, has allowed the recognition of a selective function of circulating lipoproteins in determining the risk for CHD.

There are, in fact, four major circulating lipoproteins: chylomicrons (CM), very low density (VLDL), low density (LDL) and high density (HDL) lipoproteins. Of these, HDL is directly involved in the removal of cholesterol from peripheral tissues, carrying it back either to the liver or to other lipoproteins, by a mechanism known as "reverse cholesterol transport" (RCT).

The "protective" role of HDL has been confirmed in a number of studies. Recent studies directed to the protective mechanism(s) of HDL have been focused on apolipoprotein A-I (ApoA-I), the major component of HDL. High plasma levels of ApoA-I are associated with a reduced risk of CHD and presence of coronary lesions.

Plasma ApoA-I is a single polypeptide chain of 243 amino acids, whose primary sequence is known (Brewer et al. (1978) Biochem. Biophys. Res. Commun. 80: 623-630). ApoA-I is synthesized as a 267 amino acid precursor in the cell. The major structural requirement of the ApoA-I molecule is believed to be the presence of repeat units of 11 or 22 amino acids, presumed to exist in amphipathic helical conformation (Segrest et al. FEBS Lett. (1974) 38: 247-253). This structure allows for the main biological activities of ApoA-I, i.e. lipid binding and lecithin cholesterol acyl transferase (LCAT) activation.

The apolipoprotein A-IMilano (ApoA-IM) is the first described molecular variant of human ApoA-I (Franceschini et al. (1980) J. Clin. Invest. 66: 892-900). It is characterized by the substitution of Arg 173 with Cys 173 (Weisgraber et al. (1983) J. Biol. Chem. 258: 2508-2513). The mutant apolipoprotein is transmitted as an autosomal dominant trait and 8 generations of carriers have been identified (Gualandri et al. (1984) Am. J. Hum. Genet. 37: 1083-1097). The status of a ApoA-IM carrier individual is characterized by a remarkable reduction in HDL-cholesterol level. In spite of this, the affected subjects do not apparently show any increased risk of arterial disease. Indeed, by examination of the genealogical tree it appears that these subjects may be "protected" from atherosclerosis.

The mechanism of the possible protective effect of ApoA-IM in the carriers seems to be linked to a modification in the structure of the mutant apolipoprotein, with the loss of one alpha-helix and an increased exposure of hydrophobic residues (Francheschini et al. (1985) J. Biol. Chem. 260: 1632-1635). The loss of the tight structure of the multiple alpha-helices leads to an increased flexibility of the molecule, which associates more readily with lipids, compared to normal ApoA-I.

Another very specific feature of the ApoA-IM, is its capacity to form dimers with itself and complexes with ApoA-II, in both cases because of the presence of the Cys residue.

To make possible production of sufficient quantities of ApoA-I in general, and more specifically ApoA-IM, use is made of recombinant DNA techniques, e.g. in E. coli. Thus, recombinant preparation and use of ApoA-IM, monomers as well as dimers, are disclosed in patent specifications WO-A-88/03166 assigned to Farmitalia Carlo Erba (FICE), WO-A-90/12879 assigned to Sirtori et al, as well as WO-A-93/12143 and WO-A-94/13819 both assigned to Pharmacia AB (formerly Kabi Pharmacia AB).

Use of e.g. E. coli as medium introduces certain drawbacks. Thus, endotoxins or lipopolysaccharides (LPS) are high molecular complexes associated with the outer membrane (cell wall) of gram-negative bacteria, such as E. coli, Proteus and Salmonella. Endotoxins consist of two main parts, a lipid moiety called lipid A which is embedded in the outer membrane and a polysaccharide (O-antigen) which protrudes into the environment. Lipid A is the region which elicit the toxic effect of the endotoxins, a prerequisite being the presence of the entire lipid A moiety. The polysaccharide is made up of a O-specific chain and a core. The O-specific chain projects from the core and is the outermost part of the endotoxin. The core works as a linkage between lipid A and the O-specific chain.

It is known that endotoxins must be released from the bacterial surface to cause toxic effects. This happens when the bacteria multiply, at lysis and during stress. In aqueous solutions, free endotoxins form aggregates, micelles and vesicles, with a molecular weight of about 5 kDa up to >10³ kDa.

It is known from the literature that several proteins form complexes with endotoxins. Particularly strong complexes are formed with HDL and apolipoproteins (Emancipator et al. (1992) Infect. Immun. 60: 596-601). According to Ulevitch et al. (1981) J. Clin. Invest. 67: 827-837, formation of a complex between HDL and endotoxins involve a two step mechanism, as follows:

    Endotoxins.sub.(aggregated) →Endotoxins.sub.(disaggregated)( 1)

    Endotoxins.sub.(disaggregated) +HDL→Endotoxins-HDL  (2)

This behavior has been confirmed e.g. by Munford et al (1981) Infect. Immun. 34: 835-843. There are indications suggesting that lipid A is the main factor in the complex and that the interaction involves both ionic and hydrophobic forces (Freudenberg et al. (1979) Nat. Toxins, Proc. Int. Symp. Anim., Plant Microb. Toxins., 6th, 349-354).

As already stated above, strong complexes are formed between endotoxins and HDL in general and particularly with apolipoproteins. This mechanism has been used in U.S. Pat. No. 5,128,318 assigned to the Rogosin Institute. U.S. Pat. No. 5,128,318 thus relates to HDL associated apolipoprotein containing reconstituted particles, and use thereof in removing lipid soluble materials, including endotoxins, from cells, body fluids, and the like. More particularly, U.S. Pat. No. 5,128,318 relates to a method for treating a subject for endotoxin-caused toxicity, by administering to the subject a reconstituted particle containing ApoA-I or ApoA-II, with or without cholesterol. Here, naturally, the aim is to create and maintain indefinitely the strongest possible complex, to avoid release of the endotoxins in the subject.

The complexes, strong in themselves, can be further strengthened e.g. by the presence of certain chemical compounds. Thus, deoxycholate is known to disaggregate endotoxins according to formula (1) above (Munford et al., see above and Emancipator et al., see above). The deoxycholate, then increases the binding of endotoxins to HDL according to formula (2). The result is a complex of endotoxins and HDL, which is very difficult to separate.

General methods for reducing or eliminating the effect of endotoxins are known previously. Thus, EP-A-494848 assigned to Pharmacia discloses methods for inhibiting endotoxin induced effects. A first embodiment relates to infusing a medicament containing arginine or arginine derivatives for treatment of an endotoxin induced effect, e.g. fever. A second embodiment relates to a method for removing endotoxins from water or aqueous solutions by filtering the water or aqueous solution through a bed containing immobilized arginine or an arginine derivative. To illustrate the second embodiment, tests were carried out with endotoxins from E. coli on columns containing Arginine SEPHAROSE® from Pharmacia Biotech of Uppsala, Sweden. The interaction is, however, weak and therefore much easier to separate into protein and endotoxins than would be the case with the strong complexes between apolipoproteins and endotoxins.

Anion-exchange chromatography is frequently used in the elimination of endotoxins from solutions containing proteins such as urokinase, interferon, asparaginase and albumin (Sharma (1986) Biotech. Applied Biochem. 8: 5-22). However, the interaction between the proteins and endotoxins is much weaker than the complexes formed between apolipoproteins and endotoxins.

EP-A-333474 to Mitsui Toatsu relates to a process for removing endotoxins from proteins by contacting an endotoxin-contaminated aqueous solution containing the protein with a protein adsorbent, washing the adsorbent with a solution containing an amino compound, and subsequently eluting the protein from the adsorbent. The only exemplified proteins are tissue plasminogen activator (t-PA), human serum albumin and inter-α-trypsin inhibitor. Examples of protein adsorbents are affinity, adsorption, hydrophobic and metal chelate chromatography gels.

Polymyxin B sulfate is an antibiotic polypeptide which has the ability to prevent the toxic effects of endotoxin by interaction with the lipid A moiety. Karplus and coworkers (Karplus et al. (1987) J. Immuno. Methods 105: 211-220) have used this knowledge for adsorbing endotoxins on Polymyxin Sepharose® 4B, sold by Pharmacia AB of Uppsala, Sweden. Polymyxin is, however, in itself biologically active and therefore not suitable for removing endotoxin from solutions for intravenous injection (H. Matsumae et al. (1990) Biotechn. Biochem. 12: 129-140).

PROSEP® Remtox sold by Bioprocessing Ltd. of Great Britain, is a matrix prepared for specific removal of endotoxins from low and high molecular weight substances, such as antibiotics, vitamins, enzymes, antibodies and blood products. The gel consists of a low molecular weight, non-protein, non-carbohydrate synthetic ligand.

Charged filters are capable of removing endotoxins and other negatively charged molecules from different solutions. For example, Pall of United Kingdom offers POSIDYNE™ filters, consisting of a hydrophilic nylon 66 filter medium containing quaternary ammonium groups throughout the membrane structure. The retention capacity of the filter is independent of the temperature and is optimal at a pH of 5-8 and at a low flow rate.

There are presently several methods known for reducing or eliminating the influence of endotoxins in protein solutions generally. There is, however, no existing method to overcome the strong interaction between endotoxins and ApoA or ApoE. The present invention is intended to solve this problem.

DETAILED DESCRIPTION OF THE INVENTION

An object of the present invention is to provide an efficient purifying process, for producing ApoA or ApoE with a very low content of endotoxins.

Another object of the present invention is to provide an efficient process, where the activity of the ApoA or ApoE is essentially retained.

A further object of the present invention is a process providing a high yield of ApoA or ApoE, i.e. a process with a minimal loss of product.

The objects above are met by the present invention, which relates to a process for separating endotoxins from apolipoprotein A (ApoA) or apolipoprotein E (ApoE), or variants or mixtures thereof, by contacting a first aqueous solution containing said ApoA or ApoE with a matrix containing an immobilized compound with an end group comprising two or three nitrogen atoms bonded to a carbon atom, and subsequently treating the matrix containing an immobilized compound with a second aqueous solution containing a surfactant, or by contacting a first aqueous solution containing said ApoA or ApoE with an anion-exchange matrix, and subsequently treating the anion-exchange matrix with a second aqueous solution containing a compound comprising two or three nitrogen atoms bonded to a carbon atom.

The inventors of the present invention have surprisingly found that matrices containing e.g. immobilized arginine, guanidine or histidine, can be used to strongly attach the endotoxins, and thereby the ApoA or ApoE, to the matrix. By subsequently eluting with a surfactant-containing solution, the ApoA or ApoE molecules can be released, while the endotoxins remain attached to the matrix. It is also possible to strongly attach the ApoA or ApoE and thereby the endotoxins to an anion-exchange matrix. By subsequently eluting with a solution containing e.g. urea or arginine, or salts of guanidine or histidine, the endotoxins can be released, while the ApoA or ApoE molecules remain attached to the matrix. Finally, the ApoA or ApoE molecules can be released from the matrix by increasing the ionic strength.

The selection of conditions under which to perform the present process will be governed by a desire to reach as low concentration of the endotoxins as possible, while at the same time obtain an acceptable recovery of the ApoA or ApoE.

With the present process it is possible to produce ApoA or ApoE which are substantially endotoxin-free. In the present invention, substantially endotoxin-free means a concentration below about 1 EU/mg of ApoA or ApoE. Specifically, it is possible to produce substantially endotoxin-free ApoA or ApoE which have been produced by recombinant DNA technique, more specifically in gram-negative bacteria, and even more specifically in E. coli.

With the present process it is possible to produce ApoA or ApoE with a low content of endotoxin in combination with a protein recovery of at least 70%, suitably at least 80%, preferably at least 90% and more preferably at least 95%.

The present invention also relates to use of a matrix containing an immobilized compound with an end group comprising two or three nitrogen atoms bonded to a carbon atom and a solution containing a surfactant, or an anion-exchange matrix and a solution containing a compound comprising two or three nitrogen atoms bonded to a carbon atom, for removing endotoxins from aqueous solutions containing ApoA or ApoE, or variants or mixtures thereof.

The present invention further relates to use of ApoA or ApoE produced according to the inventive process for the manufacture of a medicament comprising the ApoA or ApoE in the treatment of atherosclerosis and cardiovascular diseases.

The present invention further relates to a method for treatment of atherosclerosis and cardiovascular diseases, by administering ApoA or ApoE produced according to the inventive process in a therapeutically effective amount.

In a first embodiment, an aqueous solution containing ApoA or ApoE is loaded onto a matrix with immobilized ligands with an end group comprising two or three nitrogen atoms bonded to a carbon atom. Subsequently, the complex of ApoA or ApoE and endotoxins is separated by eluting with an aqueous solution containing a surfactant, whereby the ApoA or ApoE is released while the endotoxins remain attached to the ligands. Finally, the matrix is regenerated by washing with one or more liquids containing various combinations of e.g. NaOH, C₂ H₅ OH, HAc and NaAc.

Examples of end groups that can be used in the present invention are those containing a guanidyl group, e.g. arginine and guanidine, or a heterocyclic group, e.g. histidine. The end groups are suitably non-heterocyclic, preferably containing a guanidyl group rendering the end groups strong bases. The end group containing a guanidyl group is more preferably arginine or guanidine, most preferably arginine. The end groups can be bonded directly to the matrix. More commonly, however, the end group is bonded to the matrix through a spacer, which can be inert or exhibit additional binding capacity. Spacers well suited for the present process can be found e.g. in Arginine bonded to SEPHAROSE® and Histidine bonded to MINILEAK®.

In a second embodiment, an aqueous solution containing ApoA or ApoE is loaded onto an anion-exchange matrix. Subsequently, the complexes of ApoA or ApoE and endotoxins are separated by eluting with an aqueous solution containing a compound comprising two or three nitrogen atoms bonded to a carbon atom. In this way the endotoxins are released while the ApoA or ApoE remain attached to the ligands. Suitable examples of the nitrogen-containing compounds are urea, arginine and guanidine hydrochloride. Finally, the ApoA or ApoE are released from the matrix by increasing the ionic strength, suitably to about 0.5 up to about 2M, preferably by addition of NaCl.

Conventionally, in processes for purifying proteins the concentrations of e.g. urea and guanidine hydrochloride are kept at a minimum to avoid irreversible denaturation of the protein at issue. Surprisingly, the inventors have found that a higher than conventional concentration of the nitrogen-containing compound is advantageous when carrying out the present invention, since it facilitates the uncovering of the strong complex between endotoxins and ApoA. Thus, the concentration of urea should be in the range of from about 0.75M up to saturation at the prevailing temperature, suitably in the range of from 2.5M up to 8M, preferably from 4.5 up to 7.5M. It lies within the competence of the person skilled in the art to select the corresponding concentrations for e.g. guanidine hydrochloride and arginine.

In a third embodiment, the first and second embodiments are combined so as to give a product with a very low level of endotoxin. Thus, the aqueous solution containing ApoA or ApoE can be loaded onto an anion-exchange step whereafter the endotoxins are released by eluting with a compound comprising two or three nitrogen atoms bonded to a carbon atom. The ApoA or ApoE with a reduced concentration of endotoxins, are released from the anion-exchange matrix, and, optionally after one or more intermediate process steps such as a desalting step, the resulting ApoA or ApoE is loaded onto a matrix with immobilized compounds with end groups comprising two or three nitrogen atoms bonded to a carbon atom. Finally, the ApoA or ApoE can be released in a very pure form. The reversed sequence, i.e. a first step with a matrix containing immobilized ligands, and a second step with an anion-exchange matrix, can also be used to advantage in the present invention.

The matrices of the present invention can be soluble or insoluble in various common solvents, e.g. organic polymers soluble or insoluble in water with or without ethanol. Matrices also include e.g. filters to which ligands comprising two or three nitrogen atoms bonded to a carbon atom have been coupled.

The immobilized compounds with an end group comprising two or three nitrogen atoms bonded to a carbon atom can be supported on any inorganic or organic matrix. Thus, the matrix can be selected from various strongly hydrophilic matrices e.g. agarose matrices such as a wide variety of SEPHAROSE® matrices sold by Pharmacia Biotech of Uppsala, Sweden, organic polymer matrices such as TSK-gels sold by Tosoh Corp. of Tokyo, Japan, or highly porous organic polymer matrices sold by Per Septive Bio-systems of Boston, USA. The matrix is preferably an agarose matrix. Suitable agarose matrices in the present invention are, apart from SEPHAROSE®, MINILEAK® sold by Kem-En-Tec A/S of Copenhagen, Denmark and Bio-Gel A sold by Bio-Rad, of Brussels, Belgium. Preferably, the matrix is cross-linked allowing for a fast flow (FF) and thereby high production capacity.

Anion-exchange matrices useful in a process according to the second embodiment, are e.g. agarose matrices such as DEAE SEPAROSE® and Q SEPHAROSE® matrices sold by Pharmacia Biotech of Uppsala, Sweden. Further examples of anion-exchange matrices that can be used in the present process are Super Q-650 and Fractogel EMD DEAE-650 sold by Toso Haas of Tokyo, Japan, and Hyper D sold by Biosepra S.A. of France. The anion-exchange matrix is suitably an agarose matrix. Preferably, the anion-exchange matrix is cross-linked allowing for a fast flow (FF) and thereby high production capacity.

The present invention is advantageously used for removing endotoxins from any apolipoprotein A (ApoA) or Apolipoprotein E (ApoE), or variants or mixtures thereof. The present invention is especially suitable when the ApoA or ApoE are produced by a recombinant DNA technique in gram-negative bacteria, and in particular when they are produced in E. coli. In the present invention, the terms ApoA and ApoE include any preform or fragment, or any truncated, extended or mutated form, or any mixture of any of these forms or fragments. Preform relates e.g. to the 249 amino acid Met form of ApoA-I as disclosed in WO-A-88/03166 assigned to Sirtori et al. Other preforms are the proapolipoprotein A-Is disclosed in U.S. Pat. No. 5,059,528 to UCB as well as EP-A-308336, JP 216988/1984 and JP 252048/1987 all to Mitsubishi Chem. Ind. Fragment relates to a part of ApoA or ApoE containing at least one alpha helix, e.g. as disclosed in WO-A-93/25581 assigned to Innogenetics S.A. of Belgium. Truncated and extended forms relate to ApoA and ApoE molecules where one or more amino acid is missing or has been added, respectively, at the N and/or C terminal ends of the molecules. Suitably, from two up to eight amino acids are missing or have been added, preferably from three up to six amino acids. Mutated forms relate to ApoA and ApoE molecules where one or more amino acid has been substituted by another amino acid, e.g. ApoA-IM as disclosed in WO-A-93/12143 and WO-A-94/13819. Other mutated forms are ApoA-ISeattle (Deeb et al (1991) J. Bio. Chem. 266:13654-13660), ApoA-IYame (Takada et al (1991) J. Lipid Res. 32: 1275 ff) and a yet unnamed mutated form of ApoA-I (Matsunaga et al (1991) Proc. Natl. Acad. Sci. USA 88:2793-2797).

Human ApoE and variants thereof, are disclosed in "Human Apolipoprotein Mutants III", ed. by C. R. Sirtori et al (1993) Nato ASI Series, Springer Verlag, Berlin, II 73:81-96.

The present invention can be used to advantage for removing endotoxins from ApoA as well as ApoE. In the following description, however, use will be made of ApoA to further describe the present invention.

Known ApoAs are e.g. ApoA-I, ApoA-II and ApoA-IV. In the present invention, suitably, the ApoA is ApoA-I, or variants or mixtures thereof. Natural ApoA-I is a single polypeptide chain, composed of 243 amino acids. More suitably, the ApoA is a mutated form of ApoA-I where at least one Cys residue has been substituted for a Arg residue making formation of disulfide-linked dimer possible. In the amino acid sequence of natural human ApoA-I, Arg residues are located at positions 10, 27, 61, 83, 116, 123, 131, 149, 151, 153, 160, 171, 173, 177, 188 and 215. Of these, substitutions are preferred at one or more of positions 160, 171, 173, 177 and 188, i.e. at positions within the same alpha helix. More preferably, the Arg residue is substituted at positions 171 and/or 173.

Human apolipoprotein A-IMilano (ApoA-IM) is a naturally occurring mutated form of normal ApoA-I (Weisgraber et al. (1980) J. Clin. Invest. 66: 901-907). In ApoA-IM, one residue of the amino acid arginine (Arg 173) has been replaced by a residue of the amino acid cysteine (Cys 173). Since ApoA-IM contains one cysteine residue per polypeptide chain, it may exist as a monomer or as a disulfide-linked dimer. The molecular weight of the monomer is about 28,000 Da and for the dimer about 56,000 Da. These two forms are chemically interchangeable, and the term ApoA-IM does not, in the present context, discriminate between the two forms.

The initial concentration of endotoxin in the aqueous solutions containing ApoA, may be more than 10⁶, more than 10⁷, and even more than 10⁸ EU/mg of protein. The final concentration of endotoxin can be reduced to below about 100 EU/mg, by applying the present invention to the aqueous solutions containing ApoA. Suitably, the final concentration of endotoxin is reduced to below about 10 EU/mg, and preferably to below about 1 EU/mg, rendering the Apo A substantially endotoxin-free.

To reach low levels of endotoxin in the aqueous solutions containing ApoA, it may be necessary to recirculate the solution so that it is contacted with the matrix at least twice. It is of course also possible to use the same or different matrices in at least two consecutive steps, to reach a sufficiently low level of endotoxins. In any of these ways, it is possible to reduce the initial concentration of endotoxins by at least 10⁴, suitably by at least 10⁵, and preferably by at least 10⁶ in the process.

The matrix is normally equilibrated with a first buffer before the sample containing ApoA is loaded onto the matrix. After loading, the matrix is treated with a second buffer to elute the essentially endotoxin-free ApoA from the matrix.

In a preferred embodiment, the first aqueous solution containing ApoA further contains a surfactant. The surfactant is added in order to at least partially disaggregate the complexes between the endotoxins and ApoA before contacting said first aqueous solution with the matrix. Thus, by maintaining the first aqueous solution containing a surfactant for at least 5 min before contacting said first aqueous solution with the matrix, the interaction between the complex and the particular ligand is facilitated. Suitably, the surfactant is added and the resulting solution maintained for a period of time in the range of from 15 min up to 10 h, preferably from 30 min up to 4 h, before being contacted with the matrix.

It is a prerequisite in the first embodiment and preferred in the second embodiment, that the elution buffer contains a surfactant, suitably an anionic one such as sodium dodecyl sulfate (SDS). The surfactant enhances the effect of the compound comprising two or three nitrogen atoms bonded to a carbon atom, probably by uncovering the strong complex between endotoxins and ApoA, and subsequently separating the endotoxins from ApoA. The separation is most probably controlled by reaction kinetics, for which reason the period of time before equilibrium is reached can be substantial.

Examples of surfactants which can be used to advantage in the present invention are various bile acids or salts thereof, such as sodium deoxycholate and sodium cholate. Also, non-ionic surfactants, e.g. zero-net-charge surfactants such as polyoxyethylene sorbitan fatty esters, block co-polymers and alkyl ethoxylates, can be used to advantage in the present invention. Examples of polyoxyethylene sorbitan fatty esters are polyoxy-ethylene-(20)-sorbitan monolaurate, e.g. TWEEN® 80, and polyoxy-ethylene-(20)-sorbitan monooleate, e.g. TWEEN® 20, both sold by ICI of Great Britain. Examples of the block co-polymers are combinations of polypropyleneglycol and polyethyleneglycol, e.g. PLURONIC® sold by BASF in Germany. Examples of alkyl ethoxylates are TRITON® X-100 and Triton® X-114 sold by Union Carbide in USA.

In the present invention, surfactant also includes various lipids, which can be natural or synthetic compounds consisting of acyl carriers, such as glycerides, sphingosine, cholesterol, or derivatives or mixtures thereof, to which one or more fatty acids can be bonded. The lipids can, depending on their polarity, be divided into non-polar, polar and amphiphilic lipids. Examples of non-polar lipids are monoacylglycerides, diacylglycerides, triacylglycerides, and cholesterol. Examples of polar and amphiphilic lipids are phospholipids and glycolipids. Suitably, the polar and amphiphilic lipids are bilayer forming, such as phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylglycerol, phosphatidyletanolamine, phosphatidylserine, sphingomyelin, or mixtures thereof. The natural lipids can be produced from e.g. soybean oil, maize oil, soy lecithin and egg lecithin. Other suitable examples are synthetic and saturated or unsaturated PC:s, such as dipalmitoyl phosphatidylcholine (DPPC) and dimyristyl phosphatidylcholine (DMPC).

The concentration of surfactant in the elution buffer can be in the range of from about 2 mM up to about 200 mM, suitably from 10 mM up to 100 mM. The concentration of surfactant is preferably in the range of from 15 mM up to 50 mM.

The pH of the elution buffer is suitably in the range of from about 5 up to about 9, and preferably in the range of from 6 up to 8.

The total ionic strength of the elution buffer can be in the range of from about 0.1 up to 20 mS/cm, suitably from 1 up to 8 mS/cm, and preferably from 2 up to 5 mS/cm.

The equilibration buffer, pretreatment buffer, sample washing buffer and the elution buffer can be the same or different. The concentration of surfactant, pH and total ionic strength can be the same or different than the values given for the elution buffer.

The process can be continuous, e.g. performed on a column, or batch-wise.

It is advantageous when carrying out the present invention to make use of a high temperature, since a high temperature more readily uncovers the strong complex between endotoxins and ApoA. The temperature is, however, limited to the range where irreversable denaturation of the protein does not occur. Thus, the temperature when carrying out the present invention, can be in the range of from about 2° up to about 95° C., suitably from 15° up to 90° C., preferably from 30° up to 85° C., more preferably from 40° up to 75° C.

The following examples are intended to further illustrate the present invention, without limiting the scope of said invention.

EXPERIMENTAL Materials

Partly purified ApoA-IM in 20 mM Phosphate buffer, pH=7 was used as starting material in all performed tests. It was produced in E. coli by Pharmacia AB in Stockholm, Sweden.

The separation tests were carried out on various gels. Gel here denotes a matrix with immobilized compounds coupled thereto. Thus, tests were carried out on Arginine SEPHAROSE® 4B gel and an anion-exchange gel, Q SEPHAROSE® FF gel, both sold by Pharmacia Biotech of Uppsala, Sweden. Tests were also performed on PROSEP® Remtox gel sold by Bioprocessing Ltd. of Great Britain, and on a N66 POSIDYNE™ charged filter with filter disc, sold by Pall of Great Britain.

The analytical tests were carried out with Coatest Endotoxin and Endo-LAL, both of which are sold by Chromogenix of Molndal, Sweden.

Analytical Methods

The quantity of ApoA-IM was determined using RP-HPLC. The method is used to identify ApoA-IM in the chromatograms and calculate the yield in the different methods. The relative standard deviation (RSD) of the method is 11%.

UV-spectrophotometry was used for detection of protein in the chromatographic methods. The absorbance was measured at 280 nm.

The Limulus amebocyte lysat test (LAL test) is a specific method for measuring the concentration of endotoxins. The test is based on the primitive blood clotting mechanism in the horseshoe crab, Limulus polyphemus. The results are given in the unit EU, being equivalent to 0.1 ng of an average of various endotoxins.

The gel clot endpoint test, also known as Endo-LAL is sold by Chromogenix of Molndal, Sweden. In this test, an equal volume of LAL reagent (prepared from lysed limulus amebocyte cells) and test solution are mixed and incubated at 37° C. for one hour.

The chromogenic substrate test, also known as Coatest Endotoxin and sold by Chromogenix of Molndal, Sweden, is a relatively new, quantitative procedure that is performed on a microtiter plate and read by spectrophotometry at 405 nm. This method contains the specific enzymes which are activated in the presence of endotoxins. The activated enzymes split off para-nitro aniline (pNA) from a synthetic substrate. The released pNA form a yellow color which is proportional to the amount of endotoxins present in the reaction mixture. A standard curve is performed and the amounts of endotoxins are read off. The color development is stopped by adding acetic acid. The method is reported to give a relative standard deviation (RSD) of 25%.

EXAMPLES Example 1

Tests 1a to 1g (gels):

Five tests were performed on Arginine SEPHAROSE® (tests 1a to 1e) according to the invention. For comparison, one test was performed on Arginine SEPHAROSE® using a buffer without surfactant (test 1f). For further comparison using an elution buffer with surfactant, one test was performed on PROSEP® Remtox (test 1g) and two on POSIDYNE™ charged filters (1h to 1i).

To achieve comparable results between the various methods, tests 1a to 1g were performed in a similar way, each in one cycle. However, PROSEP® Remtox does not withstand treatment with 0.5M NaOH. Therefore, the sanitation programme in test 1g constitutes a modified program, where the concentration of NaOH is 0.1M.

All tests were performed at room temperature (about 20° C.).

Packing flow rate: 1 ml/min.

Bed height: 7.5 to 9.5 cm

Gel volume: 6 to 7 ml

Equilibration mode:

Equilibration buffer (buffer 1): 0.1M Tris-HCl+about 20 mM sodium

deoxycholate, pH=8

Flow rate in equilibration: 0.5 ml/min

Total buffer volume: 5 column volumes

Sample treatment: about 20 mM sodium deoxycholate was added to the sample and the solution was mixed 30 min prior to loading onto the column.

Loading mode:

Sample loading: about 1 mg of ApoA-IM/ml gel

Flow rate: 0.2 ml/min

Elution mode:

Elution buffer (buffer 1): 0.1M Tris-HCl+about 20 mM sodium deoxycholate, pH=8

Flow rate in elution: 0.2 ml/min

Sanitation mode

After elution, the Arginine SEPHAROSE® and PROSEP® Remtox gels were washed with 4.5 column volumes of a first washing liquid containing 50% C₂ H₅ OH and 5% HAc, followed by 4.5 column volumes of a second washing liquid containing 0.5M NaOH (tests 1a to 1g). The washing was carried out three times.

Tests 1h to 1i (charged filters):

Two tests were performed on POSIDYNE™ charged filters (tests 1h to 1i) for comparison with the process according to invention. The tests were carried out in one cycle each.

In the two tests the filters were placed in a filter disc, a pump was connected and air was removed from the system.

Equilibration mode

Flow rate: 1 ml/min

Duration: 40 min

Equilibration buffer (buffer 1) in test 1h: 0.1M Tris-HCl, pH=8

Equilibration buffer (buffer 2) in test 1i: 0.1M Tris-HCl+about 20 mM sodium deoxycholate, pH=8

Sample treatment: 4 ml buffer 1 in test 1h and 4 ml buffer 2 in test 1i were added to the samples and the solutions were mixed 30 min prior to loading onto the filter system.

Loading mode:

Samples: 1.2 mg of ApoA-IM/ml of sample solution, total volume 1 ml

Loading: The samples were circulated in the filter system for 30 min, whereupon the product was collected.

The filters were thrown away after each test.

The endotoxin values were analyzed with the LAL chromogenic substrate method (tests 1a to 1i). The concentration of endotoxins before treatment according to the invention, was between 10⁶ and 10⁷ EU/mg in all tests.

The results are evident from the following Table.

                                      TABLE I                                      __________________________________________________________________________     One-step processes for reducing the concentration of endotoxins in ApoA.                                  EU/mg in                                                                             Reduction                                     Test no.                                                                              Method      Recovery (%)                                                                           end product                                                                          (times)                                       __________________________________________________________________________     Average                                                                               Arginine SEPHAROSE ®                                                                   53      900   1 × 10.sup.3                            values with surfactant                                                         from 1a-1e                                                                     1f     Arginine SEPHAROSE ®                                                                   15      8 × 10.sup.3                                                                   1 × 10.sup.3                                   without surfactant                                                      1g     PROSEP ® Remtox with                                                                   63      1 × 10.sup.5                                                                   1                                                    surfactant                                                              1h     POSIDYNE ™ charged                                                                      68      5 × 10.sup.5                                                                   2                                                    filter without surfactant                                               1i     POSIDYNE ™ charged                                                                      77      4 × 10.sup.4                                                                   30                                                   filter with surfactant                                                  __________________________________________________________________________

As is evident from the Table, the present process is efficient in reducing the concentration of endotoxins attached to an ApoA. As is also evident from the Table, the absence of a surfactant in the elution buffer gives poor recovery and a higher concentration of endotoxins in the end product.

Example 2

The conditions in tests 2a to 2d coincided with those of tests 1a to 1g of Example 1, except that the concentration of Tris-HCl was 20 mM. The flow rate was 10 cm/h. All tests were performed at room temperature (about 20° C.).

The concentration of endotoxins before treatment according to the invention, was between 10⁶ and 10⁷ EU/mg in all tests. The recovery was at least 90%.

The results are evident from the following Table.

                  TABLE II                                                         ______________________________________                                         One-step process according to the invention, where the endotoxin values        were analyzed with the LAL gel clot endpoint method.                                                                   Reduc-                                 Test Loading  Loading   Gel     EU/mg in                                                                               tion                                   no.  mg/ml gel                                                                               EU/ml gel volume (ml)                                                                            end product                                                                            (times)                                ______________________________________                                         2a   1.2      >10.sup.6 ; <10.sup.7                                                                    26      >50; <100                                                                              >2 × 10.sup.4 ;                                                          <10.sup.5                              2b   0.9      >10.sup.5 ; <10.sup.6                                                                    40      >5; <10 >2 × 10.sup.4 ;                                                          <10.sup.5                              2c   0.8      >10.sup.6 ; <10.sup.7                                                                    40      >10; <20                                                                               >1 × 10.sup.5 ;                                                          <10.sup.6                              2d   2        >2 × 10.sup.6 ;                                                                    373     >3.3; <6.6                                                                             >3 × 10.sup.5 ;                                <2 × 10.sup.7       <2 × 10.sup.6                    ______________________________________                                    

As is evident from the Table, the first embodiment of the present process is efficient in reducing the concentration of endotoxins attached to an ApoA.

Example 3

Tests were performed on a Q SEPHAROSE® FF gel (tests 3a and 3b). The gel volume was about 0.371 and the flow rate in all steps 36 cm/h. The flow was downwards in steps 1 to 6, while the flow was upwards in step 7.

Step 1. Sanitation mode

Prior to preequilibration, the Q SEPHAROSE® gel was washed with 41 of a washing liquid containing 1.0M NaOH (solution 1).

Step 2. Preequilibration mode:

a) Preequilibration buffer (buffer 2): 0.2M Na₂ HPO₄ ×12 H₂ O, pH=8.0

Total buffer volume: 3 l

b) Distilled water

Total water volume: 5 l

c) The concentration of remaining endotoxins was measured according to the LAL gel clot endpoint test: <0.125 EU/ml

Step 3. Equilibration mode:

Equilibration buffer (buffer 3): 0.01M Na₂ HPO₄ ×12 H₂ O, 6M urea, pH=8.0

Total buffer volume: 10 l

Step 4. Loading mode

Total sample volume: 2.55 l

Step 5. Sample washing mode:

Sample washing buffer (buffer 3): 0.01M Na₂ HPO₄ ×12 H₂ O, 6M urea, pH=8.0

Total buffer volume: 0.5 l

Step 6. Elution mode:

Elution buffer (buffer 4): 0.01M Na₂ HPO₄ ×12 H₂ O, 0.1M NaCl, 6M urea, pH=8.0

Total buffer volume: 8 l

Elution with a linear gradient 0-100% (distilled water and buffer 4) in 200 min

Step 7. Regeneration mode:

Regeneration buffer (buffer 5): 2M NaCl

Total buffer volume: 0.75 l

The results are evident from the following Table.

                  TABLE III                                                        ______________________________________                                         Anion-exchange gel (Q SEPHAROSE ® FF gel) used according the               invention for reducing the concentration of endotoxins in ApoA.                                           EU/mg in                                                                              EU/mg                                              Loading              starting                                                                              in end                                                                               Reduction                              Test no.                                                                             (mg/ml gel)                                                                              Recovery (%)                                                                              material                                                                              product                                                                              (times)                                ______________________________________                                         3a    1.4       77         2.4 × 10.sup.3                                                                  <1.3  1.9 × 10.sup.3                   3b    2.8       46         43     <0.3  1.5 × 10.sup.2                   ______________________________________                                    

As is evident from the Table, the second embodiment of the present process is also efficient in reducing the concentration of endotoxins attached to an ApoA.

Example 4

One test was performed on an Arginine SEPHAROSE® gel (test 4a) according to the invention, in two cycles. Another test was performed on a Q SEPHAROSE® FF gel (step 1) followed by an Arginine SEPHAROSE® gel (step 2) (test 4b) also according to the invention. The conditions in tests 4a and 4b coincided with those of Examples 1 and 3, as appropriate. All tests were performed at room temperature (about 20° C.).

The results are evident from the following Tables.

                  TABLE IV                                                         ______________________________________                                         Process with two consecutive Arginine SEPHAROSE ® steps, and a             process with an initial anion-exchange step followed by an Arginine            SEPHAROSE ® step, both according to the invention. The endotoxin           values given in Tables IV to VI were analyzed with the LAL gel clot            endpoint method.                                                                                                        Reduc-                                Test Method    Method    Total   EU/mg in                                                                               tion                                  no.  Step 1    Step 2    recovery (%)                                                                           end product                                                                            (times)                               ______________________________________                                         4a   Arg.-Seph.                                                                               Arg.-Seph.                                                                               81      <14     4 × 10.sup.6                    4b   Anion-    Arg.-Seph.                                                                               76       <8     10.sup.6                                   exchange                                                                  ______________________________________                                    

                  TABLE V                                                          ______________________________________                                         Step 1                                                                               EU/mg in starting                                                                          Recovery EU/mg in end                                                                            Reduction                                  Test no.                                                                             material    (%)      product  (times)                                    ______________________________________                                         4a    >10.sup.6 ; <10.sup.7                                                                      87       >10.sup.3 ; <2 × 10.sup.3                                                         >2 × 10.sup.3 ; <                                                        9.5 × 10.sup.3                       4b    >10.sup.6 ; <10.sup.7                                                                      84       >90, <180                                                                               >10.sup.4 ; <6 × 10.sup.4            ______________________________________                                    

                  TABLE VI                                                         ______________________________________                                         Step 2                                                                                                  Reduction                                             Test no.      Recovery (%)                                                                              (times)                                               ______________________________________                                         4a            93         >150                                                  4b            90          >10                                                  ______________________________________                                    

As is evident from the Tables, a two-step process according to the invention provides a means to reduce the concentration of endotoxins attached to an ApoA to very low levels.

Example 5

Tests 5a and 5b (with urea)

Tests were performed on a DEAE SEPHAROSE® FF gel (tests 5a and 5b). The gel volume was about 114 l, the column height 15 cm and the flow rate in all steps about 50 cm/h (test 5a). The gel volume was about 85 l, the column height 30 cm and the flow rate in all steps about 50 cm/h (test 5b). Tests 5a and 5b were performed at room temperature (about 20° C.).

Step 1. Sanitation mode

a) The DEAE SEPHAROSE® gel was washed with a washing liquid containing 1.0M NaOH (solution 1).

b) Distilled water

c) The concentration of remaining endotoxins was measured according to the LAL gel clot endpoint test: <0.125 EU/ml

Step 2. Equilibration mode:

Equilibration buffer (buffer 2): 0.03M Tris, 8M urea, 1 mM dithiothreitol (DTT), pH=7.5

Step 3. Loading mode

Step 4. Sample washing mode:

Sample washing buffer (buffer 2): 0.03M Tris, 8M urea, 1 mM dithiothreitol (DTT), pH=7.5

Step 5. Elution mode:

Elution buffer (buffer 3): 0.055M Tris, 8M urea, 1 mM dithiothreitol (DTT), pH=7.5

Step 6. Regeneration mode:

Regeneration buffer (buffer 4): 2M NaCl

Test 5c (without urea)

One test was performed without urea present, for comparison with the present invention. The remaining conditions and concentrations were identical with those of test 5a.

The results are evident from the following Table.

                  TABLE VII                                                        ______________________________________                                         Anion-exchange gel (DEAE SEPHAROSE ® FF gel) used according the            invention for reducing the concentration of endotoxins in ApoA. The            endotoxin values given in Table V were analyzed with the LAL gel clot          endpoint method.                                                                                          EU/mg in                                                                              EU/mg                                              Loading              starting                                                                              in end                                                                               Reduction                              Test no.                                                                             (mg/ml gel)                                                                              Recovery (%)                                                                              material                                                                              product                                                                              (times)                                ______________________________________                                         5a    3         37         10.sup.8                                                                              10    10.sup.7                               5b    3         54         10.sup.8                                                                              2 × 10.sup.3                                                                   5 × 10.sup.5                     5c    3         0          --     --    --                                     ______________________________________                                    

As is evident from the Table, the second embodiment of the present process is efficient in reducing the concentration of endotoxins attached to an ApoA also on a large scale. In the absence of urea, the recovery of ApoA is undetectable, for which reason the concentration of endotoxins could not be measured either.

The eluate from the DEAE SEPHAROSE® FF gel in test 5b, was brought to a column containing 60 ml of DEAE SEPHAROSE® FF gel, the bed height being 30 cm. The column was equilibrated with the elution buffer (buffer 3) according to tests 5a and 5b. The sample was loaded and run straight through the column, being eluted with the elution buffer (buffer 3). The second step of test 5b was performed at room temperature (about 20° C.).

The results are evident from the following Table.

                  TABLE VIII                                                       ______________________________________                                         Process with two consecutive DEAE SEPHAROSE ® steps, both                  according to the invention. The endotoxin values given in Table VI were        analyzed with the LAL gel clot endpoint method.                                                                        Reduc-                                                         Total   EU/mg in                                                                               tion                                   Test no.                                                                             Step 1   Step 2   recovery (%)                                                                           end product                                                                            (times)                                ______________________________________                                         5b    Anion-   Anion-   38      <2      >5 × 10.sup.7                          exchange exchange                                                        ______________________________________                                    

                  TABLE IX                                                         ______________________________________                                         Step 2                                                                                                    EU/mg in                                                                              EU/mg                                              Loading              starting                                                                              in end                                                                               Reduction                              Test no.                                                                             (mg/ml gel)                                                                              Recovery (%)                                                                              material                                                                              product                                                                              (times)                                ______________________________________                                         5b    6         70         2 × 10.sup.3                                                                    <2    >10.sup.3                              ______________________________________                                    

As is evident from the Tables, a two-step process according to the invention provides a means to reduce the concentration of endotoxins attached to an ApoA to very low levels. 

We claim:
 1. A process for purifying apolipoprotein A (ApoA) or apolipoprotein E (ApoE), or variants or mixtures thereof, comprising (a) contacting a first aqueous solution comprising said ApoA or ApoE and endotoxins with a matrix comprising an immobilized compound with an end group comprising two or three nitrogen atoms bonded to a carbon atom for attaching the endotoxins to the matrix, and (b) subsequently treating the matrix comprising the immobilized compound with a second aqueous solution comprising a surfactant for releasing the ApoA or ApoE while the endotoxins remain attached to the matrix.
 2. A process according to claim 1, wherein the surfactant is anionic.
 3. A process according to claim 1, wherein the ApoA or ApoE are produced by recombinant DNA technique in gram-negative bacteria.
 4. A process according to claim 1, wherein the matrix is an agarose matrix.
 5. A substantially endotoxin-free ApoA or ApoE produced by recombinant DNA technique and purified according to the process of claim
 1. 6. A process according to claim 1, wherein, either before or after contacting the first aqueous solution with the matrix comprising an immobilized compound and treating the matrix comprising the immobilized compound with the second aqueous solution, the first aqueous solution is contacted with an anion-exchange matrix, and the anion-exchange matrix is then subsequently treated with an aqueous solution comprising a compound comprising two or three nitrogen atoms bonded to a carbon atom, after which the ApoA or ApoE is released from the anion-exchange matrix.
 7. A process according to claim 1, wherein the first aqueous solution further comprises a surfactant.
 8. A process according to claim 7, wherein the first aqueous solution is maintained for a period of time in the range of from 15 min up to 10 h, before contacting said first aqueous solution with the matrix comprising an immobilized compound.
 9. A process according to claim 1, wherein the ApoA is ApoA-I, or variants or mixtures thereof.
 10. A process according to claim 9, wherein the ApoA-I is ApoA-IMilano.
 11. A process according to claim 1, wherein the end group contains a guanidyl group.
 12. A process according to claim 11, wherein the end group is arginine or guanidine.
 13. A process according to claim 1, wherein, after treating the matrix, the second aqueous solution has a concentration of endotoxins reduced by at least 10⁴ as compared with a concentration of endotoxins in the first aqueous solution.
 14. A process according to claim 13, wherein, after treating the matrix, the concentration of endotoxins in the second aqueous solution is reduced by at least 10⁶ as compared with the concentration of endotoxins in the first aqueous solution.
 15. A process for purifying apolipoprotein A (ApoA) or apolipoprotein E (ApoE), or variants or mixtures thereof, comprising (a) contacting a first aqueous solution comprising the ApoA or ApoE and endotoxins with an anion-exchange matrix for attaching the ApoA or the ApoE, respectively, to the matrix, (b) subsequently treating the anion-exchange matrix with a second aqueous solution comprising a compound comprising two or three nitrogen atoms bonded to a carbon atom for releasing the endotoxins while the ApoA or ApoE remains attached to the matrix, and (c) then releasing the ApoA or ApoE from the matrix.
 16. A process according to claim 15, wherein the anion-exchange matrix is treated with a second aqueous solution comprising a surfactant and urea, arginine or a salt of guanidine.
 17. A process according to claim 15, wherein the ApoA or ApoE are produced by recombinant DNA technique in gram-negative bacteria.
 18. A process according to claim 15, wherein the matrix is an agarose matrix.
 19. A substantially endotoxin-free ApoA or ApoE produced by recombinant DNA technique and purified according to the process of claim
 15. 20. A process according to claim 15, wherein, before or after contacting the first aqueous solution with the anion-exchange matrix and treating the anion-exchange matrix with the second aqueous solution, the first aqueous solution is contacted with a matrix comprising an immobilized compound with an end group comprising two or three nitrogen atoms bonded to a carbon atom and the matrix comprising the immobilized compound is subsequently treated with an aqueous solution containing a surfactant.
 21. A process according to claim 15, wherein the ApoA or the ApoE are released from the matrix by contacting the matrix with a solution having an increased ionic strength.
 22. A process according to claim 15, wherein the ApoA is ApoA-I, or variants or mixtures thereof.
 23. A process according to claim 22, wherein the ApoA-I is ApoA-IMilano.
 24. A process according to claim 15, wherein the compound in the second aqueous solution contains a guanidyl group.
 25. A process according to claim 24, wherein the compound in the second aqueous solution is arginine or guanidine.
 26. A process according to claim 15, wherein, after the ApoA or ApoE is released from the matrix, the ApoA or ApoE has a concentration of endotoxins reduced by at least 10⁴ as compared with a concentration of endotoxins in the first aqueous solution.
 27. A process according to claim 26, wherein, after the ApoA or ApoE is released from the matrix, the ApoA or ApoE has a concentration of endotoxins reduced by at least 10⁶ as compared with the concentration of endotoxins in the first aqueous solution.
 28. A process according to claim 15 wherein the first aqueous solution further comprises a surfactant.
 29. A process according to claim 28, wherein the first aqueous solution is maintained for a period of time in the range of from 15 min up to 10 h, before contacting said first aqueous solution with the anion-exchange matrix.
 30. A process according to claim 28, wherein the surfactant is anionic. 